silicon nanowire bandgap query

[ramkrishna]

Dear Sir ,
        I wish to let you know that we are trying to find out the band gap of Hydrogen passivated rectangular Silicon nanowire. In this context there are few suggestions and I am extremely grateful for their suggestions (and the scripts attached).
However, the following scripts does not tend to produce the actual band gap as is generally observed in the literature. 
I would be extremely grateful if you correct/modify/suggest me how to make changes in the scripts attached.

Thanking you,

Ramkrishna Ghosh

[zh]

Maybe, most of the band gap calculations of silicon nanowires in literature were done with the first-principles, rather than the Huckel model. As the size of silicon nanowire in your case seems large, you may try the LCAO calculator with the smallest basis set to achieve a higher accuracy than that of the Huckel model.

[ramkrishna]

Dear Sir
          I have checked the all possible ways (for large as well as for smaller system) to get the accuracy but it does not give any proper result  and I am working on that for last few months but whenever I ask in forum, I never get any appropriate answer . Can you please provide me a proper script so that I can work on this?

Regards
Ramkrishna

[zh]

The attached script files are modified for the LCAO calculator with SZP basis set. Please try them.

[nori]

Dear ramkrishna,

First of all, both structures you configured are inadequate.
In the case of SiNW_1.py, you should define the structure as not a bulk but a molecule.
In the case of silicon_wire.py, the length of C vector of the unit cell is wrong.
The correct one is "5.4306" angstrom.

Could you try "silicon_wire-1_modified.py" and verify that the appropriate calculation can be done with it?

[nori]

I've figured it out that the passivation of dangling bonds in the Si nanowire is incomplete.
That's why you can't obtain proper result with "silicon_wire-1_modified2.py".
Sorry for it.

I modified the script again and confirmed that reasonable band structure can be obtained.
Could you try  "silicon_wire-1_modified3.py" ?

[ramkrishna]

Dear Sir ,
        Thank you for your valuable reply . Please let me know the script to do the Hydrogen passivation of dangling bonds so that I can do it properly  in lower size like  1X1nm^2 or 2X2nm^2 also . Please also let me know what should I do for changing the bond length along X and Y direction.

Regards
Ramkrishna

[ramkrishna]

Dear Sir
      I am waiting for your replies. Please help me.

Regards
Ramkrishna

[nori]

Please let me know the script to do the Hydrogen passivation of dangling bonds

I've made the script to do it with Custom Builder but it's at my office now.
My office is closed until 8th May that's why I will able to give it to you next week.

what should I do for changing the bond length along X and Y direction.

Which do you mention, Si-H bond or Si-Si bond?

[ramkrishna]

Dear Sir,
        I will wait for that and for changing the bond length I want to mention Si-Si bond.

Regards
Ramkrishna

 

[nori]

for changing the bond length I want to mention Si-Si bond.

Perhaps you can make it by the following steps:
1. drop bare silicon nanowire onto Builder,
2. switch coordinate type to fractional,
3. change the unit cell size against x or y direction.

[ramkrishna]

Dear Sir,
        I have got the script for the Hydrogen passivation from your post in "Scripts, Tutorials and Application" and applying that I have got a reasonable band structure and band gap value for higher cross section (5X5 nm^2)  but for lower cross section (like 3X3 nm^2) it gives almost zero band gap again but it should be around 2eV according to the literature. I can't understand whether there is any limitation of the software for lower cross section or not. Please find the attachment and suggest me how to overcome this problem.

Regards
Ramkrishna

[Anders Blom]

In this particular script, the Si atom in the corner with smallest XYZ coordinates is not hydrogenated properly. Maybe there are others too, I just noticed that one since it's easy to spot. This creates a dangling bond which may change the electronic structure, at least a little bit.

There is no limitation in the software that makes certain sizes of system behave differently. However, depending on the computational parameters or model used, the results may of course vary to other studies.

[nori]

I can't check your structure now but it's expected that the bare Si nanowire you prepared would be something wrong.
You should prepare a bare Si nanowire in which all Si-Si bond length are about 2.35angstrom in order to achieve proper sp3 passivation with the script.

[ramkrishna]

Dear Sir,
        I have made the bare Si nanowire by the following mathod :
custom->nanowire->wire direction(100)->radius 10A-> (100) surface energy 0.1
Now it is difficult to check form the structure that every Si-Si bonds are minimum 2.35 angstrom so that I can get proper sp3 passivation. Please suggest me, how can I achieve this ?

Regards
Ramkrishna